Optical waveguides for display devices of this type typically release guided light from an output part by a process of diffraction for viewing, whereby guided light is diffracted within the waveguide to a direction which prohibits total internal reflection thereby rendering the waveguide no longer able to retain the guided light within it. Such light simply exits the waveguide without further propagation within the waveguide, and is thereby visible to a viewer. A reversal of this process forms the means for diffractively injecting light into such a waveguide to a direction which enables total internal reflection within the waveguide for guided propagation therealong towards an output part of the waveguide. FIG. 1 schematically shows an example of this.
A planar (e.g. slab) waveguide 2 provides flat opposite surfaces (3, 4) for guiding light rays 8 along the waveguide between the opposing surfaces by successive total internal reflection (TIR) at alternate surfaces internally. An input 7 light ray is directed into the waveguide at a low angle of incidence which is insufficiently oblique to either of the opposing surfaces of the waveguide to permit TIR there. An input diffraction grating 5 is positioned upon a surface of the waveguide to receive the input light and to diffract the received ray in a direction forming an angle of incidence to the opposite surfaces of the waveguide internally which permits TIR. Guiding of the light ray 8 ensues until the guided light is incident upon an output diffraction grating 6 arranged upon the same surface 3 of the waveguide bearing the input grating 5. The output grating diffracts the received ray in a direction forming an angle of incidence to the opposite surface 4 of the waveguide internally which does not permit TIR, and light ray 9 is output from the waveguide for viewing.
However, the diffraction gratings typically used to implement this input/output process are inherently dispersive thus limiting the field of view (FOV) and the colour bandwidth available from such waveguides. As an example, a full colour image consisting of red, green and blue light, having a 35 degrees FOV may be injected into a diffractive grating waveguide. The input diffraction grating can only be optimised for one particular colour. Thus, if the input diffraction grating is optimised for green light, a substantial portion of the red light will fail to diffract as desired (diffraction will be evanescent) while a substantial portion of the blue light will be diffracted to an angle which fails to provide the total internal reflection required within the waveguide and thus fails to reach the eye of the user.
The invention desirably provides means and methods which may be used to alleviate this.